1. Field of the Invention
The present invention relates to a process for producing an ion exchange membrane. More particularly, it relates to a novel process for producing an ion exchange membrane wherein a combination of a specific base material, a specific polymerizable monomer material and a specific polymerization method, is employed.
2. Description of the Prior Art
Ion exchange membranes such as cation exchange membranes having cation exchange groups such as sulfonic acid groups or carboxylic acid groups, or anion exchange membranes having anion exchange groups such as quaternary ammonium groups, are proposed to be used for a wide range of applications, for instance, as diaphragms for the electrolysis of various materials, as membranes for electrodialysis, as membranes for diffusion dialysis, as diaphragms for fuel cells or as membranes for treating various waste solutions. For an ion exchange membrane to be practically useful, it is generally desired that an ion exchange resin be supported in a form of a film on a base material primarily with a view to providing adequate mechanical strength. As a method for supporting an ion exchange resin in a form of a film on a base material, it is known to laminate a base material such as cloth on an ion exchange membrane, for instance, by lining or embedding the cloth on the ion exchange membrane by heat-pressing. Also, various proposals have been made on a method wherein a polymerizable monomer is impregnated as it is or, if required, after being partially polymerized, into the base material, and then polymerized, if necessary, followed by a reaction for introducing ion exchange groups.
In the process for producing an ion exchange membrane comprising impregnating base material with a polymerizable monomer and polymerizing the monomer, if necessary, followed by a reaction for introducing ion exchange groups, it is possible that, depending upon the type of the base material, the affinity between the base material and the ion exchange resin is poor, and consequently, the mechanical strength and electrochemical properties of the resulting ion exchange membrane are inadequate. Particularly when the base material is made of an olefin polymer such as polyethylene or polypropylene, or a fluorinated olefin polymer such as tetrafluoroethylene, trifluorochloroethylene or vinylidene fluoride, it is expected that an ion exchange membrane having excellent heat resistance and chemical resistance can be obtained. Nevertheless, in such a case, there will be the above-mentioned problem if the conventional method for polymerization is employed. With an aim to solve such a problem, it has been proposed to graft-polymerize the polymerizable monomer with the base material by using a method of irradiating ionizing radiation as the means for polymerization, as disclosed in Japanese Examined Patent Publications No. 30136/1982 and No. 14047/1984, and U.S. Pat. No. 4,414,090.
Styrene, chloromethylstyrene and divinylbenzene have been known as polymerizable monomers used for the production of ion exchange membranes. However, the present inventors have found that when such polymerizable monomers are used in combination with a base material made of an olefin polymer or a fluorinated olefin polymer, there will be the following problems.
As disclosed in the above-mentioned Japanese Examined Patent Publication No. 30136/1982, it is necessary to employ a method in which a specific polymer layer having a graft-polymerization degree of from 3 to 80% by weight is formed on a base material either by subjecting the base material to a certain pretreatment such as irradiation of ionizing radiation and then impregnating it with a specific monomer solution to be grafted, followed by polymerization, or by impregnating the base material with a specific monomer solution, followed by polymerization by the irradiation of ionizing radiation, and then an ion exchange resin is supported by means of this polymer layer.
On the other hand, according to Japanese Examined Patent Publications No. 8857/1981 and No. 34014/1981 based on a discovery of a peculiar phenomenon of polypropylene in connection with the radiation graft polymerization, it is required to employ a pre-irradiation method or short time low temperature co-existence irradiation method under specific conditions. Thus, in the production of a cation exchange membrane by using styrene and divinylbenzene or in the production of an anion exchange membrane by using chloromethylstyrene and divinylbenzene, it is hardly possible to obtain an ion exchange membrane having adequate mechanical strength either by the deterioration of the base material or the peeling of the film-forming layer